WLANs, based on the IEEE 802.11 standards, have conventionally been used for ordinary Internet services such as web browsing, file transfers and electronic mail. However, with the emerging usage of real time multimedia applications such as voice over IP (VoIP) telephony, these same WLAN networks can also be used as infrastructure for enabling such applications. WLANs can give clients the ability to “roam” or physically move from place to place without being connected by wires. In the context of WLANs the term “roaming” describes the act of physically moving between access points (APs). One issue in the area of WLANs relates to the ability to maintain an IP-connection while roaming.
FIG. 1 is a block diagram of a conventional wireless local area network (WLAN). The WLAN 1 of FIG. 1 includes wireless clients 2, 4, a first subnet (A) 10, a wireless switch 12, access points (APs) 14, 16, a second subnet (B) 20, a wireless switch 22, access points (APs) 24, 26 and layer 3 routers 34, 36. The router 34 is coupled to the wireless switch 12. The wireless switch 12 supports the first subnet (A) 10 and is coupled to the access points (APs) 14, 16. The access points (APs) 14, 16 have IP addresses within the first subnet (A) 10. The router 36 is coupled to the wireless switch 22. The wireless switch 22 supports the second subnet (B) 20 and is coupled to the access points (APs) 24, 26. The access points (APs) 24, 26 have IP addresses within the second subnet (B) 20. The clients 2, 4 are wireless devices which physically move around the WLAN 1, and communicate with an IP network via the access points (APs) 14, 16 and access points (APs) 24, 26, respectively.
FIG. 1 illustrates the concept of layer 2 roaming and the concept of layer 3 roaming in the WLAN. A layer 2 network is defined as a single IP subnet and broadcast domain, such as the first subnet (A) 10, while a layer 3 network is defined as the combination of multiple IP subnets and broadcast domains, such as the first subnet (A) 10 and the second subnet (B) 20.
Layer 2 refers to the data link layer of the Open Systems Interconnection (OSI) communication model. The data link layer is concerned with moving data across the physical links in the network. In a network, the switch is a device that redirects data messages at the layer 2 level, using the destination Media Access Control (MAC) address to determine where to direct the message. In the context of the IEEE-802 LAN standards, the data link layer contains two sublayers called the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer. The data link layer ensures that an initial connection has been set up, divides output data into data frames, and handles the acknowledgements from a receiver that the data arrived successfully. The data link layer also ensures that incoming data has been received successfully by analyzing bit patterns at special places in the frames.
Layer 2 roaming occurs when a client moves far enough away from its AP such that its radio associates with a different AP in the same subnet. The client disconnects from one Access Point (AP) and re-connects to another AP in the same subnet (broadcast domain) where several APs use the same Service Set Identifier (SSID). A client continuously listens to nearby APs and can decide to roam if it finds an AP with the same SSID and stronger signal or is experiencing too much loss with the current AP. To initiate a layer 2 roam, the client sends an associate (or reassociate) request to the new AP. It may disassociate from the old AP, or the old AP may notice the client is no longer there.
IEEE's 802.11f Inter Access Point Protocol (IAPP) addresses roaming between Access Points (APs) inside client's home subnet and assures constant IP-connectivity in this case. With layer 2 roaming, APs inside a given subnet share the same Extended Service Set (ESS), and although the physical point of attachment (the AP) changes, the client is still served by the same Access Router. Because the original and the new AP offer coverage for the same IP subnet, the device's IP address is still valid after the roam and can remain unchanged. For example, when the roams within the first subnet (A) 10, the IP address of the client will remain the same.
After the client successfully roams, LAN traffic for the client can be relayed through the new AP. However, because the scalability of subnets is limited by the number of APs and clients that can be supported within a given subnet, in some situations the client roams to a new AP in a different or foreign subnet supported by another wireless switch. Because the client cannot be identified by its original home IP address anymore, a new IP address is required for the routing the client's IP data. Consequently, any on-going connections can be disrupted and IP connectivity can be lost. For applications like wireless VoIP phones or streaming applications, this is not acceptable.
Layer 3 refers to the network layer of the Open Systems Interconnection (OSI) multilayered communication model. The network layer is concerned with knowing the address of the neighboring nodes in the network, selecting routes and quality of service, and recognizing and forwarding to the transport layer incoming messages for local host domains.
Layer 3 roaming occurs when a client moves from an AP within its home IP subnet, such as the first subnet (A) 10, to a new AP within a foreign IP subnet, such as the second subnet (B) 20. This foreign IP subnet has a different Basic Service Set (BSS) than the home IP subnet. The client disconnects from one AP and reconnects or re-associates with another foreign AP in a foreign IP subnet outside its home IP subnet. In this re-association, the client is supposed to be served by a different access router (through the foreign AP), which bares a different IP address, while the client itself preserves its original IP address. At that point, the client would no longer have an IP address and default gateway that are valid within the foreign IP subnet. Therefore, if no other protocol is implemented to address an L3 roam, the client will not able to send/receive IP packets from/to its current location. As a result, active IP sessions can be dropped because IP-connectivity is lost.
To prevent existing data sessions or voice calls from failing because the remote client can no longer reach the local client, processes called “IP handoff” or “L3 handover” can be used to preserve the IP traffic to/from the client after such re-association with the foreign AP. Because this process is not addressed by current IEEE nor Wi-Fi standards, important functions, such as preservation of the client's IP connectivity upon a layer 3 handover, have yet to be standardized.
Nevertheless, some vendors of WLANs have developed solutions which can allow layer 3 roaming to occur by providing mechanisms for a client to obtain a new IP address. For instance, if the client roams across a boundary between the first subnet (A) 10 and the second subnet (B) 20 and a Dynamic Host Configuration Protocol (DHCP) is enabled on the client, then the client can use DHCP to obtain a new IP address of the second subnet (B) 20.
However, layer 3 traffic re-routing requires more than updating MAC address tables and ARP caches. Many applications require persistent connections and drop their sessions as a result of inter-subnet roaming. Network layer devices such as routers and layer 3 switches must somehow be told to forward IP packets to the client's new subnet. To provide session persistence, mechanisms are need to allow a client to maintain the same Layer 3 address while roaming throughout a multi-subnet network. Otherwise, many applications will timeout trying to reach the client's old IP and must be reconnect with the client's new IP.
One way to support layer 3 roaming in WLANs is via an open IETF standard called Mobile IP. Mobile IP provides one solution for handling the L3 movements of clients regardless of the underlying layer 2 technology.
In the context of Mobile IP, the client is referred to as a mobile node (MN). In the description that follows, these terms are used interchangeably. Mobile IP uses a Home Agent (HA) to forward IP packets to a Foreign Agent (FA) in the client's new subnet. The HA and FA advertise themselves using the ICMP Router Discovery Protocol (IRDP). The Foreign Agent periodically advertises its presence wirelessly and waits for a solicitation message from a roaming mobile node. When a Mobile IP-enabled client roams to a new subnet, it must discover and register itself with a nearby FA. The registration process for such a node is triggered by a wireless registration request (after the 802.11 association is completed) issued by the MN. The FA forwards that request to that client's original HA. Wired messages can then be exchanged between the HA and the FA as well as with binding table updates. An acknowledgment can then be sent wirelessly to the MN.
If the request is accepted, a tunnel is established between the HA and FA to relay incoming packets sent to the client's original IP address. The HA serves as the anchor point for communication with the wireless client. It tunnels packets from Corresponding Nodes towards the current address of the MN and vise versa. Outbound packets are routed back through the tunnel from the FA to HA, and then on to their destination.
Although Mobile IP preserves subnet connectivity for roaming clients, it can result in sub-optimal routing and longer roaming delay. As noted above, the wireless client must first regain over the air connectivity with its new FA before the Agent Discovery Phase is launched. This can result in considerable reconnection time which increases latency. Furthermore, the registration process involves wire line and wireless communication. The amount of packet loss and the significant delay introduced during these procedures make the method unsuitable for many WLAN application, such as VoIP over 802.11 or streaming over 802.11.
Notwithstanding these advances, as new applications emerge and are implemented, such as VoIP over 802.11, changes to the WLAN deployment are required. For example, coverage-oriented deployments must move to capacity-oriented deployments characterized by low user to AP ratio and more APs in a given coverage area. The move to capacity-oriented deployments emphasizes the need for techniques that allow clients to roam across subnets and roaming domains.
There is a need for layer 3 roaming techniques which can allow a client to roam across different IP subnets of a WLAN while preserving the client's original IP-connection and original IP address. It would be desirable if such techniques could allow the client to perform a seamless and smooth L3 handoff between APs of different IP subnets, while maintaining an active session without losing IP connectivity. It would be desirable if such techniques could enable routing of IP data to/from the client's current foreign subnet to their original IP address and home subnet even though the client is currently in a foreign subnet. It would also be desirable to provide layer 3 roaming techniques which can eliminate the need to re-key during re-authentication. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.